Dashes

Dashes--typed as two unspaced hyphens--are like detour signs. They tell you that you're still on your way, but you'll have a delay. You place pertinent but not essential information or commentary inside dashes or parentheses.

Dashes tell the reader additional information in an emphatic way. (Parentheses) can be used for this purpose but dashes are a more dramatic way to set off the information.)

My mother-in-law suddenly became ill--she turned red, clutched her throat, and began to wheeze--and all I could think was that I was killing her.

There was only one outstanding pianist--Gonzalez.

The cat--with its eerie light eyes and pure white coat barely visible--stayed in our memories like an apparition.

Dashes set off an abrupt turn of thought within a sentence.

I served my new mother-in-law a cake with ground walnuts in it and then--good grief!--discovered she is deathly allergic to walnuts.